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# How does an eyepiece effect F/ratio?

44 replies to this topic

### #1 nmoushon

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Posted 13 February 2018 - 11:31 AM

New to observing here. I remember reading that what ever EP you use will effect the F/ratio of your scope but can't seem to find it. So basically if you have a F/6 scope then you "X mm" EP the effective F/ratio is now "Y". Or something like that. Can someone explain to me how this works and the equation behind it? Thanks.

### #2 ascii

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Posted 13 February 2018 - 11:47 AM

Eyepieces have no effect on the focal ratio of a scope.  Is there something in particular beyond that you wanted to know about the interactions?

Focal ratio is the ratio of the focal length of the scope to the scopeâ€™s aperture.

f# = F / D

Edited by ascii, 13 February 2018 - 11:49 AM.

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### #3 MartinPond

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Posted 13 February 2018 - 12:15 PM

The cuasality is sort of reversed.

Some eyepiece designs don't work as well in a shorter (say, F/5) barrel, but perform

fine in a longer telescope (say, F/7 or more)...

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### #4 nmoushon

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Posted 13 February 2018 - 12:17 PM

Eyepieces have no effect on the focal ratio of a scope.  Is there something in particular beyond that you wanted to know about the interactions?

Focal ratio is the ratio of the focal length of the scope to the scopeâ€™s aperture.

f# = F / D

Im not talking about the scope itself, sorry. I'm talking about the apparent views you see through the EP.

So by using a 20mm EP my apparent view through the EP is = to X f/ratio but if I switch to a 40mm EP the apparent f/ratio changes. In this case to be faster. At least thats what I read.

### #5 OleCuss

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Posted 13 February 2018 - 12:31 PM

Eyepieces have no effect on the focal ratio of a scope.  Is there something in particular beyond that you wanted to know about the interactions?

Focal ratio is the ratio of the focal length of the scope to the scopeâ€™s aperture.

f# = F / D

Im not talking about the scope itself, sorry. I'm talking about the apparent views you see through the EP.

So by using a 20mm EP my apparent view through the EP is = to X f/ratio but if I switch to a 40mm EP the apparent f/ratio changes. In this case to be faster. At least thats what I read.

The focal ratio of the OTA is fixed.  You can change the effective focal ratio by using a telecompressor (focal reducer) or teleextender (something like a Barlow) but the OTA's focal ratio is still the same.

You may be thinking about the field of view?  Assuming the eyepiece is appropriately chosen, the FOV tends to get smaller if you choose an eyepiece with a shorter focal length.

The best way to calculate the true FOV of an eyepiece is to divide the field stop of the eyepiece by the focal length of the OTA - and then multiply the result by 57.3 degrees.

You can make an approximation of the FOV by taking the AFOV of the eyepiece and dividing that by the magnification of the eyepiece/telescope combination.  I do not consider this to be as accurate, but sometimes it is really difficult to determine the field stop of an eyepiece.

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### #6 areyoukiddingme

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Posted 13 February 2018 - 12:34 PM

Hmm, well if a barlow effect increases the focal length, by definition the focal ratio also increases.

I think of my F5 scope as a F5.75 with Paracorr inserted. And that's b/c of the barlow effect.

This seems to suggest that barlows stuck on the bottom end of eyepieces (or in there by design) give the top elements of the eyepieces a slower focal ratio than they would otherwise get.

### #7 starcam

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Posted 13 February 2018 - 12:40 PM

If you have a f/ratio of 5, a 2x barlow will make it a f10. a 3x barlow will make it a f15 ratio etc...effectively

### #8 nmoushon

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Posted 13 February 2018 - 12:44 PM

If a barlow or reducer changes the f/ratio of the scope why doesnt an EP? i.e 40mm acting like a reducer and a 3mm acting like a barlow. Obviously not a dramatic of a change but still.

I'm going to have to keep digging for where i read it. Maybe I read it wrong.

### #9 jallbery

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Posted 13 February 2018 - 12:47 PM

An F/ratio, by definition, is the focal length of the telescope divided by the diameter of the objective.  As such, it only applies to the telescope.

The one place I can think of where people occasionally use f/ratio in conjunction with using eyepieces is when doing eyepiece projection.  This is a photographic technique in which you use an eyepiece to project an image onto the sensor of camera rather than placing the sensor at the focal plane of the the telescope.  There are calculations that allow you to calculate the effective f/ratio of the resulting system so that you can calculate the duration of the exposure.  This depends not only on the focal length and objective size of the telescope and the focal length of the eyepiece, but also on the distance between the eyepiece and the sensor.

What we do often calculate that is similar to what I think you are asking about are exit pupils.  Take the eyepiece focal length and divide it by the telescope's F/ratio to compute the exit pupil.   The larger the exit pupil, the brighter the view.   Few people can accommodate an exit pupil bigger than 7mm (although if you are using a refractor, things work fine: you just effectively stop down the telescope; on CATs and Newts, though, this can make the central obstruction too hard to ignore).  And extremely small exit pupils (less than 0.5mm) tend to provide unsatisfyingly dim views that strain the limits of our eyes and telescopes.

See more on exit pupils at https://www.astronom...t-pupils_t.aspx

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### #10 jallbery

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Posted 13 February 2018 - 01:00 PM

If a barlow or reducer changes the f/ratio of the scope why doesnt an EP? i.e 40mm acting like a reducer and a 3mm acting like a barlow. Obviously not a dramatic of a change but still.

I'm going to have to keep digging for where i read it. Maybe I read it wrong.

A barlow or reducer doesn't change the F/ratio of a telescope: it only changes the effective F/ratio from the perspective of the eyepiece (or camera).   It's equally valid to look as the barlow as part of the eyepiece side of the system.  In this way of thinking, a 2X barlow doesn't double the focal length of the telescope, it halves the focal length of the eyepiece (and the eyepiece's field stop).  So a 2X barlow turns a 10mm eyepiece into an effective 5mm eyepiece.

>>40mm acting like a reducer and a 3mm acting like a barlow

But relative to what?  We don't visually use telescopes at prime focus.

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### #11 OleCuss

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Posted 13 February 2018 - 01:02 PM

The telecompressor and teleextender do not change the focal ratio of the OTA.  They change the effective focal ratio with which your eyepiece or sensor is working.  IOW, the focal ratio of the telecompressor and the OTA combination is different from the focal ratio of the OTA for the purposes of calculations.

The eyepiece is not a reducer or a teleextender.  The eyepiece is an eyepiece and does not reduce or extend the focal length of the OTA.

### #12 nmoushon

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Posted 13 February 2018 - 01:09 PM

An F/ratio, by definition, is the focal length of the telescope divided by the diameter of the objective.  As such, it only applies to the telescope.

The one place I can think of where people occasionally use f/ratio in conjunction with using eyepieces is when doing eyepiece projection.  This is a photographic technique in which you use an eyepiece to project an image onto the sensor of camera rather than placing the sensor at the focal plane of the the telescope.  There are calculations that allow you to calculate the effective f/ratio of the resulting system so that you can calculate the duration of the exposure.  This depends not only on the focal length and objective size of the telescope and the focal length of the eyepiece, but also on the distance between the eyepiece and the sensor.

What we do often calculate that is similar to what I think you are asking about are exit pupils.  Take the eyepiece focal length and divide it by the telescope's F/ratio to compute the exit pupil.   The larger the exit pupil, the brighter the view.   Few people can accommodate an exit pupil bigger than 7mm (although if you are using a refractor, things work fine: you just effectively stop down the telescope; on CATs and Newts, though, this can make the central obstruction too hard to ignore).  And extremely small exit pupils (less than 0.5mm) tend to provide unsatisfyingly dim views that strain the limits of our eyes and telescopes.

See more on exit pupils at https://www.astronom...t-pupils_t.aspx

Coming from the imaging side this makes more sense. Maybe the person was trying to describe the exit pupil and was using f/ratio to describe it. So my mind went straight to what an imager thinks of for f/ratio.

Still cant find where I was discussing this. Oh well.

### #13 starbase25

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Posted 13 February 2018 - 01:21 PM

New to observing here. I remember reading that what ever EP you use will effect the F/ratio of your scope but can't seem to find it. So basically if you have a F/6 scope then you "X mm" EP the effective F/ratio is now "Y". Or something like that. Can someone explain to me how this works and the equation behind it? Thanks.

https://www.cloudyni...se-the-f-ratio/

The effective focal ratio can only be changed by using a barlow lens or an aperture mask.

Edited by starbase25, 13 February 2018 - 01:27 PM.

### #14 Peter Besenbruch

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Posted 13 February 2018 - 03:17 PM

If you have a f/ratio of 5, a 2x barlow will make it a f10. a 3x barlow will make it a f15 ratio etc...effectively

Technically, no. The scope remains an f5. Practically, I can see why you would think that way, particularly if the Barlow is a good one.

### #15 bobito

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Posted 13 February 2018 - 03:25 PM

The only place I recall hearing anything similar is with eyepiece projection photoprahpy.  As described here:  http://www.astronomy...ection-formula/

It has the calculations for factoring f ratio for different EPs and how far they are from the camera chip.

### #16 Steve Daniel

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Posted 13 February 2018 - 04:02 PM

I seem to recall another case where eyepieces can affect f-ratio: SCTs. The explanation went like this: SCTs focus by moving the primary mirror relative to the corrector, which changes the focal length. Using an eyepiece, for example, that requires lots of out-focus, versus one that requires lots of in-focus, will result in different f-ratio. They effect may be small-to-negligible, but is mathematically present. I believe the effect may be more pronounced with varying diagonals with different effective lengths of light path.

...if Iâ€™m off-base, someone smarter than me please set the record straight!
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### #17 OleCuss

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Posted 13 February 2018 - 05:04 PM

I seem to recall another case where eyepieces can affect f-ratio: SCTs. The explanation went like this: SCTs focus by moving the primary mirror relative to the corrector, which changes the focal length. Using an eyepiece, for example, that requires lots of out-focus, versus one that requires lots of in-focus, will result in different f-ratio. They effect may be small-to-negligible, but is mathematically present. I believe the effect may be more pronounced with varying diagonals with different effective lengths of light path.

...if Iâ€™m off-base, someone smarter than me please set the record straight!

The explanation has some merit, but is still technically incorrect.

The eyepiece does not change the focal ratio of the SCT.  In order to use the different eyepieces the astronomer is using the focuser to change the distances which does, indeed change the focal length a little.

But it is not the eyepiece changing the focal length.  It is a design feature/flaw of the SCT which results in the change when the focuser is used.

### #18 Starman1

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Posted 13 February 2018 - 05:14 PM

New to observing here. I remember reading that what ever EP you use will effect the F/ratio of your scope but can't seem to find it. So basically if you have a F/6 scope then you "X mm" EP the effective F/ratio is now "Y". Or something like that. Can someone explain to me how this works and the equation behind it? Thanks.

The only scopes affected are moving mirror catadioptrics, like SCTs and MCTs.

The typical 8" SCT has its focal length lengthened by 31mm for every 10mm the eyepiece moves back from the scope.

Since the position of the focal plane behind the scope does not change, but the position of the focal plane in the eyepiece does, the mirror is moving to accommodate differing focus

positions for each eyepiece.

Let's say your eyepiece set has a range of focal planes from 0.25" below the shoulder to 0.38" above the shoulder, a common variation.

That means the position of the focal plane behind the scope will have to vary by 0.63" to accommodate all your eyepieces.

That would change the focal length of the 8" SCT by 3.1 x 0.63 = 1.95". or from 80" to 81.95", or from f/10 to f/10.2.

That is really negligible, so essentially, can be ignored when figuring out the focal length of the scope.

The 9.25" has a lower magnification factor on its secondary, so the focus change will be even smaller, even less of an issue--essentially not worth thinking about.

Refractors or newtonian reflectors don't have their focal lengths changed even one iota by the eyepiece.

The change of diagonal or visual back on a catadioptric will have more effect than changing the eyepiece.

Edited by Starman1, 13 February 2018 - 05:17 PM.

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### #19 starbase25

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Posted 13 February 2018 - 05:35 PM

If you have a f/ratio of 5, a 2x barlow will make it a f10. a 3x barlow will make it a f15 ratio etc...effectively

Technically, no. The scope remains an f5. Practically, I can see why you would think that way, particularly if the Barlow is a good one.

That's why it is called "effective focal ratio" The barlow makes the light cone different, so in essence, it "effectively" changes the f-ratio.

"Effective" is the key wording here.

Edited by starbase25, 13 February 2018 - 05:37 PM.

### #20 MitchAlsup

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Posted 13 February 2018 - 06:06 PM

If you have a f/ratio of 5, a 2x barlow will make it a f10.

But it remains with the coma of an F/5 scope.

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### #21 Asbytec

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Posted 13 February 2018 - 06:18 PM

"A barlow or reducer doesn't change the F/ratio of a telescope: it only changes the effective F/ratio from the perspective of the eyepiece (or camera)."

I think your correct above when discussing the effective focal ratio as it relates to eyepeice projection. I think you nailed the OP question.

However, I disagree with your quote above. A reducer or barlow changes the solid angle of the light cone. The focal ratio of the system is directly related to the solid angle subtended by the light cone and not necessarily the focal length. An f5 with a 2x Barlow and an f10 will present the same subtended solid angle at the telescopes focal plane.

The solid angle of the converging light cone provides for the image surface brightness and the image scale by the effective focal length. It's as if the primary actually had a longer focal length.

Edited by Asbytec, 13 February 2018 - 06:23 PM.

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### #22 jallbery

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Posted 13 February 2018 - 08:34 PM

The focal ratio of the system is directly related to the solid angle subtended by the light cone and not necessarily the focal length. An f5 with a 2x Barlow and an f10 will present the same subtended solid angle at the telescopes focal plane.

The solid angle of the converging light cone provides for the image surface brightness and the image scale by the effective focal length. It's as if the primary actually had a longer focal length.

I agree.   In fact, I'm not quite sure what part of my statement you are disagreeing with, but I probably read it the way I meant it...

When I say that the barlow only changes the focal ratio "from the perspective of the eyepiece (or camera)," I'm agreeing with your statement: in the case of an F/5 scope with a 2X barlow, the eyepiece will "see" an F/10 light cone comparable to that of a scope with double the focal length.   Of course, the scope still has the characteristics of an F/5 scope (e.g., if it is a Newt, it still has the coma of an F/5 Newt).  However, I did go on to say:

It's equally valid to look as the barlow as part of the eyepiece side of the system.  In this way of thinking, a 2X barlow doesn't double the focal length of the telescope, it halves the focal length of the eyepiece (and the eyepiece's field stop).

If you look at it this way, the scope is still an F/5 scope at its native focal length, but the eyepiece has been transformed to one with half the focal length, half the fieldstop, and comparable performance (in terms of off axis performance) to an eyepiece of the same design at F/10.

I tend to prefer to look at barlows this way:  they transform my eyepieces rather than my telescopes.

Edited by jallbery, 14 February 2018 - 01:09 AM.

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### #23 Asbytec

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Posted 13 February 2018 - 11:22 PM

Jallberry, yea, I'm not sure where we're crossing lines on semantics.

I got the impression you said the image of an f5 scope is always an f5 image and the Barlow is part of the eyepeice.

My semantic assertion is the f5 telescopic image formed on the scope's focal plane has the qualities of f10 with a 2x Barlow.

This is because the Barlow steepens the converging solid angle (etendue) despite the native f5 focal length and the primary's aperture.

Edited by Asbytec, 13 February 2018 - 11:25 PM.

### #24 Starman1

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Posted 14 February 2018 - 12:54 AM

As far as the eyepiece is concerned, the Barlow converts the f/5 light cone to an f/10 light cone where the angle of the rays hitting the eyepiece is concerned.

So a Barlow often cleans up induced astigmatism in an eyepiece.

In that sense, it is as if the Barlow is doubling the focal ratio of the scope.

But, a Barlow also does not change any aberrations present in the primary mirror, so coma and astigmatism in the optics are not corrected.

In that sense, it is as if the Barlow is merely a part of the eyepiece and halving the focal length of the eyepiece.

Maybe that clears up the back-and-forth, here.

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### #25 Jon Isaacs

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Posted 14 February 2018 - 02:14 AM

Lots of good stuff in this thread..

The system of a telescope and an eyepiece is "afocal" system which means the system itself has no focal ratio or focal length, it can be defined by exit pupil and magnification.

Jon

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